1. Field of the Invention
The present invention relates to an image memory and, more specifically, to an image memory particularly suitable for an image processing apparatus.
2. Description of Related Art
FIG. 1A shows a schematic structure of a conventional field image memory IC (for example, MN4700 produced by Matsushita Electronics Corporation). The memory IC comprises a shift register WSR for writing and a shift register RSR for reading capable of high speed operation provided separately on a data input portion and on a data output portion of a memory device M. In serial writing operation, data from a serial access bus 2 is held in a register in the first stage of the shift register WSR for writing, and when the next data is inputted, the data which has been held is shifted to the neighboring register. The operation is successively repeated until data for 8 pixels are stored in the shift register WSR for writing, then the data for 8 pixels are simultaneously written in the memory device M. The serial reading operation is the reversal of this operation, in which the data for 8 pixels are simultaneously read out from the memory device M to the shift register RSR for reading, and data for each pixel is successively shifted to be serially outputted to the serial access bus 2.
A dual port memory has been known which has both a random access port RP and a serial access port SP as shown in FIG. 2A (for example, .mu.PD41264C, produced by NEC Corporation). In the dual port memory, a random access port RP is connected to a random access bus 1 enabling random access to the memory device M. However, since only one shift register is provided to the serial access port SP connected to the serial access bus 2, the reading operation cannot be carried out simultaneously with the writing operation.
A conventional image memory has been developed for recording still images, and therefore sequential input/output function of the data was given priority. Meanwhile, in an image memory employed for an image processing apparatus, various processes should be carried out, such as geometrical conversion of images, generation of a figure in the memory device, an so on. However, these processing cannot be sequentially carried out, and therefore the random access function is essential in order to enable access to an arbitrary pixel in the image memory. In the image memory for the image processing apparatus, high speed transfer of image data is necessary for real time processing, so that the serial input/output must be done at high speed. The function of simultaneously inputting/outputting image data is necessary for effective use of the image memory.
In order to carry out interactive image processing such as superimposing of an image on a displayed image, reading of pixel value at a prescribed position, the serial reading function and the random access function must be simultaneously used.
Although a commercially available field image memory IC has high speed serial input/output function, the reading and writing of images can be carried out only by the block unit and lacks random access function. Accordingly, it is not suitable for an image memory of an image processing apparatus requiring access of pixel unit. Meanwhile, although the dual port memory is capable of random access, it has only one shift register for serial access, therefore the input and output operations cannot be simultaneously done. Therefore it is not suitable for effective use of the image memory.